As described in “Electronic Warfare in the Information Age” by D Curtis Schleher (1999) published by Artech House, Inc., constant false alarm rate (CFAR) processing adjusts a radar detection threshold in the presence of clutter residues. Cell-averaging CFAR is one of the more prominent CFAR techniques and a well-known form of cell-averaging CFAR is disclosed in M. M. Finn and R. S. Johnson, Adaptive detection mode with threshold control as a function of spatially sampled clutter-level estimates, RCA Review, Vol. 30 (1968), pp. 414-465. Clutter returns may vary both spatially and temporally, and in the example of sea clutter this variation may be caused for example by variations in wind and ocean currents. Clutter will generally be a function of the angle of view.
An outline of a known, basic form of CFAR is illustrated in FIG. 1. This adaptive control system for use in radar apparatus receives an input xi (i=1, 2, . . . , N) which is a succession of N samples of radar return signals, representing successive range cells in a scene. Each of these return signals may be optionally the result of “within beam integration” (WBI), whereby samples in each range cell from successive radar pulses are integrated. The control system comprises a range cell-averaging CFAR filter which is used to estimate the local clutter mean level surrounding the cell under test. This mean level estimate zi (i=1, 2, . . . , N) is multiplied 7 by a first variable factor α, the threshold multiplier, and is used to set a threshold t against which the range cell under test is compared. The output from this threshold in comparator 8 is either a 0 or a 1 for each successive range cell tested.
In the example shown in FIG. 1, the cell under test 3 is at the centre of a group of M++2G+1 cells. Each cell 1, 2, 3, 4, 5 of the G cells 2, 4 on either side of the cell under test is ignored, whilst the M/2 cells 1, 5 at each end are averaged to produce the mean level estimate zi. This can be written mathematically as:
      z    i    =            1      M        ⁢          (                                    ∑                          j              =                                                                    -                    M                                    /                  2                                -                G                                                    j              =                                                -                  G                                -                1                                              ⁢                      E                          i              +              j                                      +                              ∑                          j              =                              G                +                1                                                    j              =                                                M                  /                  2                                +                G                                              ⁢                      E                          i              +              j                                          )      
The CFAR configuration which determines M is of course selectable depending upon the specific application.
The appropriate value of the first variable factor α is a complicated function of the clutter amplitude statistics, the clutter spatial characteristics and the configuration of the cell averaging filter. In general, the appropriate value of α is a continuously varying function of radar look direction and the range and depression angle relative to the radar of the cell under test. The CFAR control system measures the false alarm rate at the output OUT of the detection threshold and continuously adapts a in a closed loop control system as shown, using the adaptive control circuitry 6, to achieve the desired false alarm rate under changing conditions.
Measurement of false alarm rate requires an adequate number of independent samples of the process to be taken. However, the clutter characteristics and hence the appropriate value of α will change with range and look direction. Samples taken from too large an area would not allow local control of α, whilst samples taken from too small an area would give poor estimates of the false alarm rate. In practice, the CFAR system controls a using returns from a series of annuli centred on zero range. The range extent of each annulus is such that an appropriate false alarm rate can be estimated using a single range spoke from the radar. This allows a to be updated at successive pulse intervals as the radar scans.
In order to accommodate typical variations of conditions with radar range, it is desirable to have as many CFAR annuli as possible and as discussed above this restricts the false alarm rate that can be measured accurately.